Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves are most common because they reside in the left side of the heart where pressures are the greatest.
Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve which uses a ball and cage arrangement or a pivoting mechanical closure to provide unidirectional blood flow. The other is a tissue-type or "bioprosthetic" valve which is constructed with natural-tissue valve leaflets which function much like a natural human heart valve's, imitating the natural action of the flexible heart valve leaflets which seal against each other to ensure the one-way blood flow. In both types of prosthetic valves, a biocompatible cloth covered suture ring on the valve body (mechanical) or stent (tissue-type) provides a platform for attaching the valve to the annulus of the particular valve being replaced.
The valves of the heart separate chambers therein, and are each mounted in an annulus therebetween. The annuluses comprise dense fibrous rings attached either directly or indirectly to the atrial and ventricular muscle fibers. In a valve replacement operation, the damaged leaflets are excised and the annulus sculpted to receive a replacement valve. Ideally the annulus presents relatively healthy tissue which can be formed by the surgeon into a uniform ledge projecting into the orifice left by the removed valve. The time and spacial constraints imposed by surgery, however, often dictate that the shape of the resulting annulus is less than perfect for attachment of a sewing ring. Moreover, the annulus may be calcified as well as the leaflets and complete annular debridement, or removal of the hardened tissue, results in a larger orifice and less declined annulus ledge to which to attach the sewing ring. In short, the contours of the resulting annulus vary widely after the natural valve has been excised.
Conventional placement of the valve is intra-annular, with the valve body deep within the narrowest portion of the annulus to enhance any seal effected by the sewing ring/suture combination and reduce the chance of perivalvular leakage. Surgeons report using at least 30 simple sutures or 20 mattress-type sutures to prevent leakage. Mattress sutures are more time consuming and essentially comprise double passes of the needle through the tissue with one knot.
The four valves separate each ventricle from its associated atrium, or from the ascending aorta (left ventricle) or pulmonary artery (right ventricle). After the valve excision, the annulus generally comprises a ledge extending into and defining the orifice between the respective chambers. Prosthetic valves may attach on the upstream or downstream sides of the annulus ledge, but outside of the ventricles to avoid interfering with the large contractions therein. Thus, for example, in the left ventricle a prosthetic valve is positioned on the inflow side of the mitral annulus (in the left atrium), or on the outflow side of the aortic annulus (in the ascending aorta). Besides the differing anatomies of the mitral and aortic annuluses, the pressures exerted on the attachment sutures differ as well. The highest pressures to which the sutures are subjected in use is in the backflow half of the flow cycle when the valve closes. In systole, the left ventricle contracts to push blood through the body's circulatory system and the mitral valve is forced closed by pressures of up to 140 mm Hg. Because the prosthetic mitral valve is attached on the inflow side of the annulus opposite the ventricle chamber, the sutures are placed in direct tension. In contrast, the backflow pressure of the ascending aorta on the aortic valve is much less, and in any event the back pressure pushes the prosthetic valve against the aortic annulus so that the attaching sutures are not in tension. The end result is that care must be taken so that the mitral valve is more securely attached, and pledgets are conventionally used in conjunction with sutures in both aortic and mitral implantations to avoid a "cheesewire" effect on the tissue. Pledgets are small pieces of biocompatible fabric attached to each individual suture that are positioned within the loop of the suture between the suture and the tissue to prevent the suture when placed in tension from cutting into the tissue.
Naturally, the implantation of a prosthetic heart valve, either a mechanical valve or a bioprosthetic valve (i.e., "tissue" valve), requires a great deal of skill and concentration given the delicate nature of the native heart tissue, the spatial constraints of the surgical field and the criticality of achieving a secure and reliable implantation. It is of equal importance that the valve itself have characteristics that promote a long valve life and that have minimal impact on the physiological makeup of the heart environment.
Given the uneven nature of the annuluses, the design of the sewing ring and the method with which the sewing ring is fixed into place are perhaps the most crucial aspects of prosthetic heart valve implantation. Accordingly, an optimum sewing ring design contemplates a blend between structure highly complimentary to the valve annulus tissue and a valve attachment platform that simplifies the implantation procedure for the surgeon. Although prior art sewing ring designs are widely varied and numerous, until the design of the present invention, attempts to effectively blend improved structure/tissue compatibility with a convenient "surgeon friendly" sewing platform have been largely unsuccessful.
Many prior art sewing rings are designed to take up little space so as to increase the potential orifice opening for the valve within. One example of a prior art sewing ring may be found in U.S. Pat. No. 5,397,348 to Campbell et al. which discloses a sewing ring made of a solid PTFE felt ring having a cross-sectional shape of a right triangle. The sewing ring is mounted to a mechanical valve, and one side of the ring extends perpendicular to the flow direction through the valve, thus the right triangle designation. The PTFE felt ring is enveloped by cloth that conforms to the right-triangular shape. When implanted in the mitral position as shown in FIG. 1 of the Campbell patent, the hypotenuse of the right triangle mates with the tissue in the valve annulus.
The design typified by the Campbell patent has a number of significant drawbacks. For example, the solid nature of the PTFE felt ring does not easily conform to an irregularly shaped annulus and introduces an inherent stiffness that limits the ability of the sewing ring to flex with the annulus tissue as that tissue is stressed during normal heartbeat activity. The lack of flexibility or low compliance, in turn, increases the loads exerted on the sutures used to attach the sewing ring potentially leading to leakage problems or damage to the annulus tissue. For example, unduly stiff sewing rings must be sutured in place fairly tightly to prevent perivalvular leakage between sutures. This added tension may strangle the annulus tissue and result in a decubitous ulceration.
The inherent stiffness (low compliance) also severely narrows the margin for error when selecting the appropriate size sewing ring/valve for a given patient. If the selected size is slightly too large, the inability of the PTFE felt ring to easily compress requires undue deformation of the annulus tissue in order to adequately attach the valve. Similarly, if the selected size is slightly too small, the inability of the PTFE felt ring to easily stretch results in undue tension on the tissue and sutures in order to achieve attachment. As a result, a great deal of care and accuracy by the surgeon are needed in the selection of a valve size that precisely matches the valve annulus of the patient. Unfortunately, standard sizing tools are provided in increments based on an overall orifice size, and may not be able to accurately measure a less than optimally formed annulus. The surgeon thus must use informed judgment in selecting an approximate valve size.
The combination of the stiffness in the PTFE felt ring with the right triangle shape also has drawbacks. For example, the valve annulus tissue typically does not have a cross-section which matches the linear hypotenuse, and given the inherently stiff and bulky nature of the PTFE felt ring, there is insufficient flexibility for the hypotenuse edge of the ring to bend in a manner that adequately conforms to the irregular, nonlinear shape of the sculpted annulus cross-section. This again potentially results in perivalvular leakage and tissue damage.
The stiffness/right triangle shape combination also is a limiting factor in providing adequate sewing ring cross-sectional area for suturing (or other attachment methods, e.g., stapling) the valve to the annulus tissue. The annular band of material around the periphery of the sewing ring which serves as the suturing platform is relatively narrow in a radial dimension which necessitates the use of pledgets in conjunction with the sutures. Obviously, the use of pledgets increases the complexity and time required for valve implantation. The annular band of a right triangular sewing ring is so radially narrow that the suture loop passes through a relatively thin portion of the annulus tissue near the annulus tip, and so pledgets must be used.
The implantation problem caused by narrow sewing rings is aggravated in many prior prosthetic valves by rigid structure extending outward from the valve body into the interior of the sewing ring. See, for example, the compressible stiffening ring of Campbell (U.S. Pat. No. 5,397,348). This structure further limits the placement of sutures in the sewing ring to the radially outer regions thereof. Moreover, if attempts were made to increase the annular band of the sewing ring, or to at least increase the cross-sectional angle of the hypotenuse in order to provide a larger suture platform, the solid nature of the PTFE felt ring would only cause an undesirable increase in stiffness and bulkiness. Such a result would then simply amplify the problems already discussed with regard to sewing ring stiffness and low compliance.
In view of the foregoing, it is evident that an improved sewing ring that addresses the apparent deficiencies in existing sewing rings is necessary and desired. That is, there is a need for an advanced design that improves compatibility of the ring to the annulus tissue and simultaneously simplifies for the surgeon the technique used to attach the valve.